Braking systems of heavy trucks generally use air under pressure as the operating medium. Typically, air actuators have a mechanical output which applies brake pads to the brake rotor via a lever mechanism. The lever mechanism includes an operating shaft pivotable about two bearing surfaces on a common axis to urge one or more pistons against a brake pad via a cam surface. The shaft has a lever arm acted upon by the air actuator, and may include another generally shorter arm to actuate a wear adjuster mechanism. For larger brake pads, twin pistons are provided to optimize pressure distribution on the brake pad backplate. A known braking mechanism of this type is shown in document WO2004/074705 (Knorr-Bremse).
For lighter duty applications (e.g., lighter trucks or trailers of tractor-trailer units) with smaller brake pads a single piston is sufficient, but nevertheless two bearings are typically provided. This arrangement adds to the cost and weight of the brake, complexity of machining the bearing surfaces, and also results in problems in packaging the components within the smaller brake envelope, because space at the inboard (with respect to the vehicle on which the brake is fitted) side of the brake rotor is limited. An example of a brake of this type is EP0730107 (Perrot Bremsen).
The present applicant in EP1852627 B1 proposed a solution to this problem. However, the actuation mechanism disclosed therein retains a relatively complex wear adjuster mechanism, cover plate, carrier and pad construction.
In addition, a potential issue with single piston disc brakes is the greater risk of uneven or so-called tangential taper pad wear due to the effect of turning moments generated by the frictional forces within the brake as the brake is applied. EP0641949B1 (Perrot Bremsen) discloses a channel and “dog” formation in the interface of the brake carrier and a pressure plate to counteract taper wear. However this requires complex machining of a carrier abutment surface, increasing costs, and the channel may also reduce the strength of the carrier.
The present invention seeks to overcome or at least mitigate the problems associated with the prior art.